The present invention relates to techniques for producing fuel cell separators.
A fuel battery is of a stack structure constructed from a plurality of stacked single cells as unit components. FIG. 9 is a fragmentary sectional view of a single cell 100 constituting a solid polymer electrolyte fuel battery such as disclosed in Japanese Unexamined Patent Publication No. 10-3931 (1998). The single cell 100 includes an electrolyte membrane 101, an anode 102 and a cathode 103 sandwiching the electrolyte membrane 101 therebetween, and a pair of separators 104 sandwiching the anode-cathode pair.
The pair of anode 102 and cathode 103 sandwiching the electrolyte membrane 101 therebetween form gas diffusion electrodes. The separator 104 acts as an electrical conductor for the gas diffusion electrode and also as a structure providing gas flow channels defined by a plurality of protuberances formed on a surface of its base plate 104a. A gas flow channel between the anode 102 and the separator 104 is a fuel gas channel P1, whereas a gas flow channel between the cathode 103 and the separator 104 is an oxidizer gas channel P2.
FIG. 7 is a perspective view showing an exemplary separator 104. The separator 104 is constructed such that a plurality of protuberances 104b are arranged on a surface of the base plate 104a. The separator 104 is constructed from a gas impermeable conductive member such as of a molded carbon article formed of an electrically conductive carbon material compressed for reduced gas permeability.
FIG. 8 is a sectional view showing a state where the separator 104 is molded. The separator 104 is produced by the steps of charging conductive carbon into a lower die 105 of a predetermined shape, and heating the conductive carbon under pressure with an upper die 106.
Holes 105a in the lower die 105 for forming the protuberances 104b are produced by machine work using a drill, end mill or the like.
Unfortunately, the holes 105a machined using a tool, such as drill, end mill or the like, tend to suffer an inadequate depth precision because the tool is worn to cut the holes short of depth. In addition, the worn tool results in the hole 105a with round bottom, the circumferential edge of which is also rounded. Hence, it is difficult to ensure that the protuberances 104b have constant planar top surfaces.
As a result, the molded separator 104 detrimentally has the protuberances 104b which are irregular in height and poor in flatness at top surfaces thereof. The use of such a separator 104 in the fuel cell entails a problem that the separator contacts the anode 102 or the cathode 103 on a reduced area to increase contact resistance.
In view of the foregoing, it is an object of the present invention to provide a set of molding dies, molding structure and molding process for fuel-cell separator with the protuberances of a constant height and a constant flatness at the top surfaces thereof.
In accordance with the present invention for achieving the above object, a set of molding dies for fuel-cell separator formed with a group of protuberances on its base place comprises:
a lower-die base having its surface finished to a predetermined surface roughness;
a plate disposed in intimate contact with the surface to thereby form a lower die together with the lower-die base and having a predetermined uniform thickness and a plurality of through holes extended in a direction of its thickness; and
a side frame disposed on a periphery of the plate and having a predetermined height.
In the molding dies of the above construction, the group of protuberances of the fuel-cell separator are formed by plural minor cavities defined by the plural through holes in the plate and the surface of the lower-die base. The top surfaces of the protuberances are formed by the surface of the lower-die base, which is finished to the predetermined surface roughness. Accordingly, the top surfaces of the protuberances are finished to the predetermined surface roughness. The height of the protuberances is determined by the thickness of the plate so that the protuberances have the predetermined uniform height. Thus is ensured that the protuberances have the constant height and the constant flatness at their top surfaces.
In the above set of molding dies, the through hole may be progressively increased in size from one side on the lower-die base toward the other side.
In this case, the protuberances of the molded fuel-cell separator are readily released from the through holes during the removal of the separator. This facilitates a releasing operation.
In accordance with the present invention for achieving the above object, a molding structure for fuel-cell separator comprises:
a lower-die base having its surface finished to a predetermined surface roughness;
a plate disposed in intimate contact with the surface to thereby form a lower die together with the lower-die base and having a predetermined uniform thickness and a plurality of through holes extended in a direction of its thickness;
a side frame disposed on a periphery of the plate and having a predetermined height;
a molding material for fuel-cell separator charged into a cavity having its bottom defined by the plate and its side wall surfaces defined by inside surfaces of the side frame; and
an upper die inserted in the side frame for compressing the molding material.
In the molding structure of the above construction, the molding material for fuel-cell separator is charged into the cavity with its bottom defined by the plate and the side wall surfaces defined by the inside surfaces of the side frame and then is compressed by the upper die for formation of the base plate of the fuel-cell separator. The plural protuberances of the fuel-cell separator are formed by the minor cavities defined by the plural through holes in the plate and the surface of the lower-die base. That is, the top surfaces of the protuberances are formed by the surface of the lower-die base, which is finished to the predetermined surface roughness. Accordingly, the top surfaces of the protuberances are finished to the predetermined surface roughness. The height of the protuberances is determined by the thickness of the plate, thus resulting in the predetermined uniform height. Thus is ensured that the protuberances have the constant height and the constant flatness at their top surfaces.
In accordance with the present invention for achieving the above object, a molding process for fuel-cell separator comprises the steps of:
placing a plate in intimate contact with a surface of a lower-die base finished to a predetermined surface roughness, the plate having a predetermined uniform thickness and a plurality of through holes extended in the direction of its thickness;
placing a side frame of a predetermined height on a periphery of the plate;
charging a molding material for fuel-cell separator into a cavity having its bottom defined by the plate and side wall surfaces defined by inside surfaces of the side frame;
inserting an upper die in the side frame for compressing the molding material;
heating the compressed molding material for forming the material into shape; and
releasing the molded fuel-cell separator from the upper die, side frame, lower-die base and plate.
In the above molding process for fuel-cell separator, the molding material for fuel-cell separator is charged into the cavity with bottom defined by the plate and side wall surfaces defined by the inside surfaces of the side frame and then, is heated as compressed with the upper die for formation of the base plate of the fuel-cell separator. On the other hand, the plural protuberances of the fuel-cell separator are formed by the plural minor cavities defined by the plural through holes in the plate and the surface of the lower-die base. The top surfaces of the protuberances are formed by the lower-die base surface which is finished to the predetermined surface roughness. The height of the protuberances is determined by the thickness of the plate, so that the protuberances have a uniform height. That is, the fuel-cell separator featuring the protuberances of the constant height and the constant flatness at their top surfaces is obtained by releasing a molded article from the upper die, side frame, lower-die base and plate.